The production of transportation fuels (e.g., ethanol) from biomass continues to attract interest due to the wide availability of biomass, environmental benefits, and because biofuels may be used to displace the use of fossil fuels. For example, ethanol may be blended into gasoline at predetermined concentrations (e.g., 10%).
First generation biofuels, also referred to as conventional biofuels, are made from biomass that contains sugar, starch, or vegetable oil. For example, ethanol may be produced by fermenting sugars that are easily extracted from sugar crops (e.g., sugar cane or sugar beets), or may be produced by fermenting sugars derived from starch-based feedstocks (e.g., corn grain, barley, wheat, potatoes, cassava). In fact, the diversion of farmland or crops for first generation biofuel production has led to much debate about increased food prices and/or decreased food supplies associated therewith. In addition, there are concerns related to the energy and environmental impact of these production processes.
Second generation biofuels, also referred to as advanced biofuels, wherein the biomass contains lignocellulosic material and/or is obtained from agricultural residues or waste (e.g., corn cobs, corn stover (e.g., stocks and leaves), bagasse, wood chips, wood waste), may allay some of these concerns. For example, when bioethanol produced using second generation processes (i.e., also referred to as cellulosic ethanol) is derived from agricultural waste or residue, its production should not affect the food supply. In fact, tremendous effort is currently being expended to advance cellulosic ethanol production processes.
Lignocellulosic biomass typically contains cellulose, hemicellulose and lignin, each of which is present in plant cell walls. Cellulose (e.g., a type of glucan) is an unbranched chain polysaccharide including hexose (C6) sugar monomers (e.g., glucose). Hemicellulose is a branched chain polysaccharide that may include different pentose (C5) sugar monomers (e.g., xylose and arabinose) in addition to glucose. Lignin is a complex organic polymer, which typically includes cross-linked phenol polymers. Although generally insoluble in water at mild conditions, lignin may be soluble in varying degrees in dilute acid or base alkali. The ratio and/or structure of these components may vary depending on the source of the biomass.
The production of ethanol from lignocellulosic biomass most often involves breaking down the cellulose and/or hemicellulose into the constituent sugars, which may then be fermented. Unfortunately, the cellulose, hemicellulose, and/or lignin found in lignocellulosic biomass is typically structured within the plant walls to resist degradation.
Since lignocellulosic biomass is naturally resistant to breakdown into its constituent sugars, a pretreatment step is often used to open up the structure of the material and/or to make it accessible for enzymes used to hydrolyze the cellulosic component. Some examples of pretreatments include dilute acid pretreatment, alkali pretreatment (e.g., lime), ammonia fiber expansion, autohydrolysis (e.g., hot water extraction that does not require the addition of acid or base), steam explosion, organic solvent, and/or wet oxidation.
One type of pretreatment is sulfur dioxide (SO2)-catalyzed steam pretreatment. Sulfur dioxide is a gas, which when dissolved in water, is referred to as sulfurous acid. Sulfur dioxide and/or sulfurous acid may be a suitable catalyst for acid-catalyzed steam pretreatment since it may produce a more digestible substrate and/or may produce less/fewer inhibitors relative to other acid pretreatments, such as dilute sulfuric acid (H2SO4) catalyzed pretreatments. In addition, sulfur dioxide catalyzed pretreatment may be effective at relatively low temperatures and/or reaction times (e.g., relative to dilute sulfuric acid pretreatments).
Although sulfur dioxide catalyzed pretreatment offers some advantages over dilute sulfuric acid catalyzed pretreatments, the use of sulfur dioxide is often considered expensive and/or is associated with environmental concerns. For example, in a conventional batch pretreatment, the sulfur dioxide and/or sulfurous acid is added to lignocellulosic biomass, pressurized and/or heated (e.g., with steam), and then depressurized and discharged from the reactor. Once the reactor has been emptied it may be loaded with additional lignocellulosic biomass and sulfur dioxide/sulfurous acid. Cost and environmental concerns arise because a significant makeup amount of sulfur dioxide may be required (e.g., or generated if using sulfurous acid) for each sequential batch, which may also need to be recovered.